ox: Change `org-export-collect-footnote-definitions' signature
* lisp/ox.el (org-export-collect-footnote-definitions): Refactor
code. Change signature.
* testing/lisp/test-ox.el (test-org-export/collect-footnote-definitions):
New test.
(test-org-export/footnotes): Update test.
* lisp/ox-ascii.el (org-ascii-inner-template):
* lisp/ox-html.el (org-html-footnote-section): Apply signature change.
* etc/ORG-NEWS: Document signature change.
This change makes `org-export-collect-footnote-definitions' compatible
with `org-export-get-footnote-number' and
`org-export-footnote-first-reference-p'.